The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In a high speed serial communication link, such as the lanes of a 10 Gigabit (10 G) or 100 Gigabit (100 G) Ethernet connection, a transmitter transmits a data signal into a communication channel (channel) without an accompanying clock signal. The data signal includes a sequence of symbols, each symbol carrying information from some number of bits, such as one, two, or more bits, or in some cases fractions of bits.
The symbols are transmitted at a modulation rate expressed in baud, where one baud is one symbol per second. The duration of each symbol is known as the Unit Interval (UI).
In order to receive the data on the communication link, a receiver determines a phase and a frequency of a clock used to sample a signal received from the channel. The process of determining the phase and frequency of the clock is part of a Clock and Data Recovery (CDR) process.
The receiver may have a baud-rate architecture, meaning that the receiver only samples the signal from the channel once per UI. In a baud-rate architecture, the CDR circuit of the receiver may seek to adjust the phase and frequency of the clock so that sampling occurs in the middle of an appropriate UI for each symbol in order to maximize the Signal-to-Noise Ratio (SNR).
Because the channel coupling the transmitter to the receiver is not perfect, dispersion and reflections occur in the channel. As a result, one or more portions of the energy from a transmitted symbol may arrive at the receiver at different times.
At high symbol rates, where the range of arrival times for the energy from one symbol is greater than the UI, energy from a plurality of sequentially transmitted symbols may arrive at the receiver simultaneously. The energy from each symbol contributes to the signal value seen at the receiver, a phenomena called Inter-Symbol Interference (ISI).
ISI makes CDR more difficult, and also lowers the SNR which may lead to bit errors.